Chemical analyses are often important in evaluating the characteristics and contents of a liquid. For example, in caring for a critically ill patient, it is desirable to know the concentrations of, for example, gases (oxygen, carbon dioxide, etc.) and ions (potassium, calcium, etc.) in the patient's blood. Another example is the culturing of cells, such as in fermentation (e.g., for the production of bacterial antibiotics). In such instances, the levels of nutrients and gases in the liquid are monitored to provide a suitable environment for cell growth and the production of desired products.
Chemical sensors have been used in measuring chemical parameters in liquids. For example, U.S. Pat. No. 4,785,814 (Kane) discloses an optical probe for measuring pH and oxygen content in blood in a blood vessel within a living body. The optical probe has an elongated flexible optical fiber, the distal end of which is adapted to be inserted into a blood vessel. A membrane constructed of a hydrophilic porous material containing a pH sensitive dye is secured to the distal end of the optical fiber. This membrane receives light from the optical fiber and returns light therethrough to the proximal end of the fiber. Another example of using an optical fiber in a chemical sensor is disclosed in U.S. Pat. No. 5,176,882 (Gray et. al.). The chemical sensor described by Gray et. al. is capable of sensing more than one analyte. This sensor has two fiber optic sensor cells, one of which is for measuring a combination of ionic species. A second fiber optic sensor cell is used for measuring gaseous species. Each of the sensor cells has a membrane that is permeable to the corresponding ions and gases of interest.
Although the above chemical sensors are described as being suitable for use with physiological samples, they will not function properly in highly dispersive media (i.e., liquids containing particulate matters) such as fermentation broths, sewage treatment streams, contents of fluidised bed reactors, and slurries. A fermentation electrode calibrator has been described by Kok and Hogan (Biosensors 3, 89-100, (1987/88)). This calibrator (with an oxygen probe) is described as being suitable for use in situ in a fermentor. Scrubbing tubes directed at a face of the oxygen probe provide high velocity jets (of air or steam) for cleaning. However, the calibrator of Kok and Hogan is mechanically complex and bulky, making it unsuitable for use in a small liquid sample. What is needed is a chemical sensor of relatively simple mechanical construction for application in a liquid with particulate matters.